Hybrid electric vehicles offer reduced fuel consumption and emissions by utilizing both an electric motor/generator and an internal combustion engine and an onboard electronic control unit (ECU) programmed to vary use of each of the engine and motor/generator during different driving conditions to achieve peak efficiency.
Typical hybrid motor vehicle systems include series hybrid systems and parallel hybrid systems. In a series hybrid system, the internal combustion engine drives a generator which recharges a high voltage battery which in turn powers the electric motor/generator to power the vehicle. In a parallel hybrid system, both the electric motor/generator and the internal combustion engine may provide power to drive the vehicle. Differential gearing may be employed to allow power flow through a purely mechanical path resulting in direct or fixed ratio from the internal combustion engine, or the motor/generators may be employed to provide power to or receive power from the engine through the differential gearing, thus allowing a continuously variable speed ratio between the input and the output of the transmission. Again, the ECU controls power flow from the engine and the electric motor/generators to allow for greatest efficiency under current driving conditions. Parallel hybrids also require a high voltage battery (e.g., a 36 volt battery) that is utilized in certain driving conditions to power the motor/generator when it acts as a motor, to receive power from the motor/generator when it acts as a generator and during regenerative braking. The high voltage battery allows for a “quick start” with sufficient power to launch the vehicle from a temporary stop such as a stop light, when the ECU has to shut off the internal combustion engine for emissions purposes. Thus, hybrid motor vehicles are able to recharge the high voltage battery during driving (i.e., when the motor/generator is acting as a generator) and it generally does not require any offboard battery charging.
Most hybrid vehicles also employ a low voltage battery (e.g., a 12 volt battery) to run typical motor vehicle accessories, such as headlights, an audio system, the ECU and other electronic components. The low voltage battery may also be recharged by the high voltage battery (via an auxiliary power module (“APM”), also referred to as a DCDC converter, that converts power from the high to the low voltage) assuming the APM has been activated (which is only after the engine is running). As used herein, “activating” the APM means causing the APM to begin converting voltage from the high voltage level to a lower voltage level, or vice versa.
Different hybrid motor vehicles may utilize the high and low voltage battery systems differently in performing a first start (also referred to herein as a cold start) of the hybrid motor vehicle. “First start” or “cold start” refers to the motor vehicle being started for the first time after the motor vehicle has been completely shut off for an extended time. This is in contrast to the “quick start” described above. A first start is typically initiated by a vehicle operator inserting a key into the ignition and turning the key to a key-up or further to a key-crank position. In one configuration of a hybrid motor vehicle, the high voltage battery is used to perform the vehicle first start and the low voltage battery is retained for powering vehicle accessories but is not utilized in the first start process. In another configuration, the low voltage battery is utilized in a first start by supplying power to a starter motor to start the internal combustion engine, with the high voltage battery utilized for quick starts but not for first starts. The low voltage battery also powers the motor vehicle accessories after the first start. In yet another electrical system configuration, the starter motor initially turns the internal combustion engine to a first speed during the first start and the motor/generator is then employed in tandem with the starter motor to assist in turning the internal combustion engine to a final start speed as described in U.S. Pat. No. 6,769,389 issued Aug. 3, 2004 to Tamai et al., which is commonly assigned and is hereby incorporated by reference in its entirety.